1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring fluid level in oil wells. More particularly, the invention is concerned with a novel sensor apparatus and the method of using the same for measuring the changes in weight of the tubing in an oil well which occur as a result of changes in buoyancy of the tubing due to the rise and fall of the fluid level within the well.
2. Description of the Prior Art
Oil field pumping units can be of several types. For example, a common type of unit is the so-called "sucker rod" apparatus which uses a standard walking beam coupled with a down hole sucker rod. Pumping units other than the "sucker rod" type use either a submersible pump and motor with power being supplied to the motor from the surface or a surface-mounted motor which drives a submersible rotary pump via an elongated drive shaft.
The oil well itself typically comprises a drilled bore which is lined with a steel casing. Within this casing is a down hole tubing which, in other than the submersible pump and motor type unit, carries either the sucker rod or the drive shaft which drives the impeller of the submersible pump. In the case of the submersible pump and motor type unit, the down hole tubing carries the cables which provide power to the submersible motor.
In any of the conventional prior art oil field pumping units, if the well runs dry or if the fluid level within the well drops significantly, the load on the pump motor will decrease causing the speed of the motor to rapidly increase. Accordingly, in wells wherein fluid levels vary during pumping, control means must be provided to either turn off the pump motor when the fluid level is low, or, alternatively, to control the speed of the motor to accommodate changes in load on the motor due to the rise or fall of fluid level within the well. Without such controls, substantial damage to the pump can occur. More particularly, in the case of the submersible pump and motor, if the motor is allowed to run in the absence of fluid, potentially devastating wear and tear on the mechanical parts of the pump can quickly occur. Retrieval of the damaged or destroyed pumping unit and its repair or replacement can be quite costly. Similarly, in surface-mounted motor units, which include a down hole rotary pump and impeller, operation of the pump, when the fluid level is too low can also result in substantial damage to the system.
In addition to protecting the oil well pump from running in the absence of fluid, speed control is also necessary for the efficient operation of the pump and to avoid energy waste. For example, if the fluid within the well is not being pumped fast enough due to a rapid rise in the fluid level, means for adjusting the stroke speed of the pump is highly desirable to maintain peak production. Conversely, if the rise in fluid level within the well slows, slowing of the stroke speed of the pump is desirable.
Various types of methods and apparatus for pump control in oil wells have been suggested in the past. The most common type of prior art control system involves the use of a load cell type device in conjunction with a pump controller which generates a well card. The well card typically monitors the condition of the well and is a visual depiction of oil well conditions. In operation of this type of system the controller periodically energizes the pump for a few cycles to determine if there is sufficient fluid within the well to warrant continued operation of the pump. If the system detects the absence of fluid, the pump will immediately be de-energized. If, on the other hand the system determines that ample fluid is available for pumping, the pump will be permitted to operate until the system senses the absence of sufficient fluid level within the oil well. In actual practice after a number of periodic energization and de-energization steps occur, a trend is established and the controller functions to automatically energize and de-energize the pump based on this established trend. However, if the well conditions change radically, a new trend must be established in order to maintain peak operating efficiency and to prevent damage to pumping system. Disadvantageously, prior to the establishment of the new trend, the pump may operate for several cycles without fluid to pump, thereby exposing the pump to damage. In addition to being somewhat inefficient, the prior art load cell system cannot be used in connection with surface mounted pump systems.
As will be better understood from the description which follows, the method and apparatus of the present invention provides continuous monitoring of the fluid level within the well and interfaces with controls that maintain the correct pumping speed at all times. Additionally, in accordance with the method of the present invention, should fluid not be available to the pump, the pump will be automatically de-energized until fluid levels are sufficient for the resumption of safe pumping.
The need for continuous and precise pump control is even more important when surface-mounted or progressive cavity pumps are used as distinguished from sucker rod-type pumps. Accordingly, many of these types of prior art pumps embody a torque transducer which measures load level on the pump and when a decrease in torque occurs on the impeller shaft, the pump is automatically de-energized. Again a trend is established as the pump controller energizes and de-energizes the pump under varying conditions of fluid level. However, due to the possibility of immediate and extreme damage to the pump in the progressive cavity pump systems, it is highly undesirable to allow the pump to start up even momentarily when the fluid level within the well is too low.
Advantageously, in accordance with the method of the present invention there is a continuous monitoring of fluid level within the well and simultaneous control of pump operation will positively prevent the pump from operating when the fluid level within the well is too low. Additionally, in accordance with the method of the present invention the stroke speed of the pump is continuously controlled so as to maintain proper pumping conditions as fluid levels rise and fall within the oil well.
Another widely used prior art method for determining fluid level in an oil well involves the use of a small capillary tube which is coupled with a pressure transducer located near the top of the tubing. With this type of system, the pressure transducer measures the rise and fall of fluid within the oil well as the pressure changes in the capillary tube. This continuous monitoring of the well conditions satisfactorily avoids starting of the pumps when the fluid level is too low and also can be interfaced with the controller that will adjust the pumping speed to accommodate a rise in fluid level within the well. However, due to the fact that a very long length of very costly capillary tubing is required, this type of prior art system is quite expensive to manufacture. Further, the fragile capillary tubing is easily damaged during installation and operation. Additionally, the small capillary tubing is prone to blockage which results in faulty pressure measurements by the pressure transducer associated with the capillary tubing. Disadvantageously, when the small and fragile capillary tubing is either blocked or damaged, a specialized crew is required to take remedial action which can be both time consuming and quite costly.
The novel method and apparatus of the present invention overcomes the drawbacks of the prior art capillary tubing systems while at the same time achieving superior results. Further, the apparatus of the present invention is considerably more economical to operate, is more reliable in operation, and does not require a special crew for installation and repair since all the measurements occur at a location proximate the surface of the ground.